I just want you to love science as much as I do…

Technically speaking, you don’t have a boner. (But your dog does).

The penis. It comes in so many different shapes and sizes… and that’s just in humans. However, different species have very different penises. For instance, you may not be aware that most mammals have a bone in their penis. AN ACTUAL BONE! It is called a baculum, or os penis.

BEHOLD! A walrus baculum!

Now you may or may not have noticed that humans don’t have a penis bone- in fact, humans are the only primate species besides the spider monkey to lack this “os penis”. We are certainly the exception rather than the rule – the majority of mammals boast a baculum, including the ones we think we know well, like cats, dogs and rats. Among those species who do without a penis bone are whales, horses, rhinos, rabbits, elephants, marsupials and hyenas (whose…interesting… genitals I have covered before here). There is also a female version of the baculum, which has a rather lovely name – the baubellum, or os clitoris, but unfortunately I could find very little information on it. It seems that it has been generally accepted that the baubellum (which apparently means “little gem” in latin – awww!) is essentially an equivalent of male nipples – it is a non-functional, underdeveloped version of the functional male counterpart.

WHY HAVE A PENIS BONE?

Well, simply put, to help a male maintain an erection long enough to penetrate into a female’s reproductive tract and deliver sperm. The baculum is generally kept in the male’s abdomen until it is required, at which point abdominal muscles push it out into the penis, thus erecting it. The other function of a baculum is speed. Sliding an already-erect bone into the fleshy penis is much easier and more reliable than waiting on it to fill up with enough blood to maintain an erection long enough to deposit sperm into a female (as is the case with us ever-romantic human beings). This is of real importance in many species, as mating often has to be quick and opportunistic. It also allows for quantity over quality mating – for instance, a lion’s baculum allows him to engage in his ordinary 250-copulations-in-four-days routine. Each copulation only lasts a minute or so, but his ever-ready baculum makes it easy to get geared up for the next willing lioness shortly after his previous ejaculation.

A gentleman looking scared/intimidated by a walrus baculum (the biggest of all bacula)

So this inevitably brings us to the question of why humans are the only apes to lack a penis bone. The answer is because human males are SO AWESOME that they don’t need a freaking bone to get an erection – they can do it all by themselves with blood pressure! LOL, kidding, it actually turns out that Adam sacrificed it in order to make Eve. No, really! The following paragraph is taken from an academic paper called Congenital Human Baculum Deficiency: The Generative Bone of Genesis 2:21±23

“One of the creation stories in Genesis may be an explanatory myth wherein the Bible attempts to find a cause for why human males lack this particular bone. Our opinion is that Adam did not lose a rib in the creation of Eve. Any ancient Israelite (or for that matter, any American child) would be expected to know that there is an equal (and even) number of ribs in both men and women. Moreover, ribs lack any intrinsic generative capacity. We think it is far more probable that it was Adam’s baculum that was removed in order to make Eve. That would explain why human males, of all the primates andmost other mammals, did not have one. …

Interestingly, Biblical Hebrew, unlike later rabbinic Hebrew, had no technical term for the penis and referred to it through many circumlocutions. When rendered into Greek, sometime in the second century BCE, the translators used the word pleura, which means side, and would connote a body rib (as the medical term pleura still does).

A rib has no particular potency nor is it associated mythologically or symbolically with any human generative act. Needless to say, the penis has always been associated with generation, in practice, in mythology, and in the popular imagination. Therefore, the literal, metaphorical, and euphemistic use of the word tzela make the baculum a good candidate for the singular bone taken from Adam to generate Eve.”

Well, who’da thunk. There we have it, human males don’t have a penis bone because God took Adam’s out to make Eve. Thanks, God.

*BACK TO REALITY/SCIENCE*

Actually the real reason that humans don’t have a baculum is not entirely understood, but it is believed to be down to our mating systems and strategies. Although our closest living relatives, the great apes, possess penis bones, they are very small and it is possible that they may too eventually lose their bacula. Perhaps it’s more a question of why the other great apes still have bacula, rather than why us humans lack them. Complete loss of a baculum in humans seems to just continue a trend towards size reduction which is found among great apes. It is also thought that the presence of a baculum is associated with longer mating or perhaps just much more of it (like the lion discussed above), and the mating systems of humans did not require this additional help anymore. So just remember, the next time someone tells you they have a boner, reply with, “you don’t.”

I’m not sure if that’s ever been done Jereme! So presumably there is increased blood flow to the penis in animals who have a baculum when they are sexually aroused, but because they have evolved to rely on the bone, they would probably struggle to mate and certainly woulnd’t be able to comete with the other males who have their helping bone! They would perhaps still get a cardio-vascular erection but I would think that it would have a severe effect on their fitness (meaning they would not produce as many offspring as they otherwise would)

No, animals who have the baculum lack the capacity to produce an erection like animals who naturally lack the bone (humans, some whales, rhinos, etc). This is because mammalian species without a baculum have a seemingly unique form of hydrostatic skeleton for their genitalia. This is because unlike other soft wiggly things with hydrostatic skeletons like worms where their tissues are arranged in concentric helical patterns that give them shape, and mobility. However us fun mammalian species without baculum have our penises structural tissues in perpendicular configuration, this gives us the ability to have a penis that is able to be inflated and wont bend, unlike our worm counter parts. So long story short, the sad boneless lion would not be able to have sex because it would not have the uniquely shaped internal structures in its penis to establish the needed rigidity.
Source: http://www.ted.com/talks/diane_kelly_what_we_didn_t_know_about_penis_anatomy.html